1. Field of the Invention
The present invention relates to optical fiber amplifiers and in particular, to optical fiber amplifiers which flatten the power of optical signals which are amplified from the optical fiber amplifiers and will be output extending over a wide wavelength range and a wide optical input power range as well as a method that flattens the power of optical signals which will be output from the optical fiber amplifiers.
2. Description of Related Art
Optical fiber amplifiers include, for example, erbium-doped optical fiber amplifiers, praseodymium-doped optical fiber amplifiers, neodymium-doped optical fiber amplifiers and optical fiber amplifiers doped with other rare earth elements. For instance, production of erbium-doped optical fiber amplifiers, research into their characteristics and further development have advanced as described in the paper xe2x80x9cHIKARIZOFUKUKI TO SONO OYOxe2x80x9d (Ohm Corp. published May 30, 1992, pp. 99 to 123).
An erbium-doped optical fiber amplifier (EDFA) is an amplifier that utilizes an optical fiber with a glass fiber core doped with erbium ion (Er+3) (erbium-doped fiber). Pumping light forms an inverted population inside the EDF. When an optical signal is input as an optical input into an EDF having formed therein an inverted population, emissions occur inside the EDF amplifying these optical signals. The amplified optical signals are then output as an optical output.
FIG. 1 shows characteristics of the wavelengths and optical output power of an EDFA. In FIG. 1, the wavelength is shown in the horizontal axis (units: nm) and the optical output power is shown in the vertical axis (units: dBm) FIG. 1 shows output characteristics when optical signals of eight different wavelengths each having an optical power of xe2x88x9226 [dBm/ch] are simultaneously input into an EDFA as optical inputs and then amplified. From the output characteristic data shown in FIG. 1, it is understood that there is a difference in the optical output power for each wavelength. In other words, within the plurality of wavelengths there is a wavelength dependency output power deviation xcex94G in the output between the maximum optical output power and the minimum optical output power. This deviation xcex94G is an output (or gain) deviation.
When this EDFA is provided in multistages in a transmission system having a long span as optical amplifier repeater, the output power deviation xcex94G accumulates sequentially. Because of this, that wavelength causes the optical output signal to experience S/N degradation, and in a worst case scenario, signal loss.
Further, the wavelength dependency output power deviation of a conventional EDFA changes in response to the input state. FIG. 2 shows the relation between the wavelengths of an obtained optical output signal and optical output power when the optical input power changes to seven paths and is input into an EDFA for optical signals having eight different wavelengths. The wavelength is shown in the horizontal axis (units: nm) and the optical output power is shown in the vertical axis (units: dBm). Characteristic curves A, B, C, D, E, F and G correspond to and optical input power of xe2x88x9212, xe2x88x9214, xe2x88x9216, xe2x88x9218, xe2x88x9220, xe2x88x9222 and xe2x88x9224 (units: dBm/ch), respectively. As can be understood from the results shown in FIG. 2, the output deviations occurring in the optical output power of the EDFA differ in response to changes in the optical input power. For example, according to the results shown in FIG. 2, the optical output power close to 1532 nm is approximately 8 dBm and the output deviations of the optical output power are small for the input. However, if the wavelength expands to the 1544 nm side, the optical output power becomes smaller and is at its minimum close to 1540 nm. Then, the output deviation of the optical output power for the signal close to 1540 nm changes approximately 4 dB with respect to changes in the input power. This means that the slope of the spectrum of the optical output signal during batch amplification of multiple waveforms changes in response to changes in the input power.
Conventionally, the following two methods have been proposed as methods to reduce (compensate) the wavelength dependency output power deviation of an EDFA and flatten the output power.
The first method is a method wherein a gain equalizer comprising an interference filter and/or a fiber grating is inserted on the output side in a first stage EDFA or within or on the output side in a second or additional stage EDFA. This method cancels output deviations of EDFAs by means of providing the gain equalizer with characteristics opposite to the output characteristics which are wavelength dependent.
The second method is a method wherein improvements are made to the amplifier medium itself. For example, in this method the amplifier medium itself makes the output power flatten by adding phosphorus (P) to an EDF to create a hybrid EDF.
However, the first method that uses the above-mentioned gain equalizer has the following problems. At first, it is difficult to design a gain equalizer that has characteristics opposite to an EDFA. Next, in reality a gain equalizer only corresponds to a wavelength dependency output power deviation of one certain optical input power. In other words, it is not possible to use a gain equalizer to flatten the optical output power of a wavelength division multiplexing signal output from an optical fiber amplifier extending over the entire input power range and input wavelength region of a wavelength division multiplexing signal input to an optical fiber amplifier. For attaining the reduction of a wavelength dependency output power deviation over the entire input power range and input wavelength region by means of the first method, it would probably be necessary to use an active gain equalizer (optical component) with gain equalizing characteristics which depend on the optical input power. For example, the necessity of a movable portion to change the incident angle of the optical signal directed towards a filter for each power or the necessity of optical components to change the angle of the filter in response to an input signal make it difficult to practically use the first method reliably and with control.
The second method that utilizes an amplifier medium has the following problems. When this method is used, it is difficult to design the amplifier medium. Namely, the wavelength range in which the amplifier medium can handle amplification processing is limited. Therefore, it is difficult to provide a composition of an amplifier medium to achieve gain flattening of optical power of an optical output signal extending over a wide wavelength range. Further, the above-mentioned wavelength dependency output power deviation is dependent on the optical input power as previously described referring to FIG. 2 for the hybrid EDF doped with phosphorus (P) and another amplifier medium. Because of this, the optical input power does not allow flattening of the optical output power. In other words, uniform gain cannot be achieved.
Thereupon, the inventors of this application carried out various research and experiments to solve the above-mentioned problems. As a result, the inventors were aiming at an EDFA with amplification characteristics of optical power which were dependent on the wavelength in addition to absorption characteristics of optical power which were dependent on the wavelength as well. The inventors considered that if amplification optical fibers which actuate by amplification characteristics and absorption optical fibers which actuate by absorption characteristics were connected in series together within one transmission path as well as absorption optical fibers having absorption characteristics which allow compensation of amplification characteristics, it would be possible to equalize (flatten) the gain, namely the optical power, of each optical multiplexed output signal from the amplification optical fibers.
The object of the present invention is to provide optical fiber amplifiers which can flatten the gain of optical output signals, namely optical power, with respect to optical input signals extending over a wide wavelength range and a wide optical input power range as well as efficiently flattening the optical power.
According to a first aspect of the present invention, optical fiber amplifiers are provided which use optical fibers to amplify optical signals which enter the amplifiers from an input port and are emitted from output port. The optical fiber amplifiers include main optical amplifiers and first gain equalizers connected to the main optical amplifiers between the input port and the output port. The main optical amplifiers comprise first optical fibers which amplify the optical signals, a first pumping light supply means which supplies a first pumping light to excite the first optical fiber, and a non-reciprocal means which controls the reflection of light on the input port and the output port. The first gain equalizers comprise a second optical fiber for flattening the gain of the power of the optical signal to be emitted from the output port within a fixed wavelength range.
According to this composition, a second optical fiber that absorbs the optical power of optical signals is provided in addition to the first optical fibers which amplify the optical power of optical signals. Deviations in the optical power of optical output signals amplified by the first optical fibers, namely, wavelength dependency of output deviations (referred to as amplification output deviations) can be measured and discovered in advance. Further, deviations in the optical power of optical output signals absorbed by the second optical fibers, namely, wavelength dependency of output deviations (referred to as absorption output deviations) can also be measured and discovered in advance. Therefore, the first optical fibers and the second optical fiber that transfers absorption output deviations which can provide optimal compensation of the amplification output deviations are used in combination. When used in combination in this manner, the second optical fiber equalizes the gain of the output power of the main optical amplifiers during operation of the optical fiber amplifiers, in other words, the second optical fiber operates as a gain equalizer that flattens the output power. Because of this, it becomes possible to flatten the output power from the optical fiber amplifiers. Moreover, the first gain equalizer can be disposed in front of or in back of the main optical amplifiers.
According to the present invention, gain flattening uses the wavelength dependency of absorption characteristics of the second optical fiber to compensate for wavelength dependency output power deviations based on amplification characteristics of the first optical fiber.
In addition, the above-mentioned non-reciprocal means can be a means that combines a function to cut out pumping light or can be added thereto a separate means that cuts out pumping light as necessary.
According to a preferred embodiment of the present invention, the second optical fiber can be provided with a wavelength dependency of the absorption characteristics opposite to the amplification characteristics of the first optical fiber within a fixed wavelength range. According to this composition, gain flattening can be efficiently achieved.
According to another preferred embodiment of the present invention, a stimulated emission cross-sectional surface area of a first equivalent optical fiber may be identical to the reverse of a light absorption cross-sectional surface area of a second equivalent optical fiber when it is assumed that the first optical fiber is replaced by the first equivalent optical fiber and the second optical fiber is replaced by the second equivalent optical fiber. According to this composition, without regard to the number of main optical amplifier and first gain equalizer stages provided in the transmission path, it is possible to match the amplification characteristics of all provided main optical amplifiers with the absorption characteristics of all provided gain equalizers in such a manner to cancel each other. This makes it possible to achieve flattening of the output power from the optical fiber amplifiers.
According to another preferred embodiment of the present invention, it is preferable for the first and second optical fibers to be cut from one optical fiber produced under identical manufacturing conditions. In this manner, the wavelength dependency amplification characteristics and the wavelength dependency absorption characteristics of the first and second optical fibers are almost identical. This makes it possible to achieve efficient gain flattening, namely, flattening of the output power.
According to another preferred embodiment of the present invention, a non-reciprocal means can comprise first isolators connected between the input port and the first optical fiber and second isolators connected between the first optical fiber and the output port. The non-reciprocal means may comprise a means that cuts out pumping light as necessary, for example, adding a filter. According to this composition, the reflection of light from the input port and the output port can be controlled in addition to allowing pumping light to be cut out as necessary.
According to a preferred embodiment of the present invention, it is preferable to provide a second pumping light supply means that supplies a second pumping light to excite the second optical fiber and change the wavelength dependency of the absorption characteristics of the fiber to characteristics different from the wavelength dependency of the absorption characteristics during a non-pumping state. The adjusting of the wavelength dependency of the absorption characteristics of the second optical fiber will make it possible to achieve efficient flattening of the output power.
Furthermore, according to an embodiment of the present invention, it is preferable to provide a detection means that detects the power of the optical signal, and a pumping light control circuit that controls the power of the second pumping light from the second pumping light supply means based upon the detected power from the detection means. According to this composition, for example, the optical input signal or changes over time of the output power of the first optical fiber are allowed to track making it possible to achieve gain flattening of the output power with even more efficiency.
In this embodiment of the present invention, it is preferable to provide a prevention means that prevents the first pumping light from the first pumping light supply means from entering the second optical fiber. According to this composition, during the operation of the optical fiber amplifiers fluctuations in the wavelength dependency absorption characteristics caused by the first pumping light of the second optical fiber can be controlled. Therefore, the wavelength dependency absorption characteristics of the second optical fiber due to the signal of the first optical fiber can be utilized.
In this embodiment of the present invention, it is preferable for the first pumping light supply means to have a first optical means, for example, a first WDM (Wavelength Division Multiplexing Signal) coupler and a first pumping light source that generates the first pumping light. The first optical means can be connected to the input port, the first pumping light source and the first optical fiber, and the optical signal and the first pumping light can be optically multiplexed passing through to the first optical fiber.
Further, the first optical means is connected to the output port, the first pumping light source and the first optical fiber with the optical signal passing through to the output port side and the first pumping light passing through towards the first optical fiber.
In this embodiment of the present invention, it is preferable for the first gain equalizers to be connected between the main optical amplifiers and the output port and even further, to provide a second gain equalizer between the first gain equalizers and the output port. According to this composition, a wavelength range and/or power range with an insufficient amount of output flattening by the first gain equalizers can be compensated by using the second gain equalizer.
The second gain equalizer need not be a movable portion and can be an interference filter, a fiber grating an etalon filter or a Mach-Zehnder type filter.
In this embodiment of the present invention, it is preferable for the second pumping light supply means to have a second optical means, for example, a second WDM (Wavelength Division Multiplexing Signal) coupler and a second pumping source that generates the second pumping light. The second optical means passes an optical signal towards the second optical fiber or an optical signal from the second optical fiber in addition to passing the second pumping light towards the second optical fiber.
According to a second aspect of the present invention, optical fiber amplifiers include first and second main optical amplifiers and first gain equalizers connected to the first and second main optical amplifiers between the input port and the output port. The first and second main optical amplifiers comprise first optical fibers which amplify the optical signals, a first pumping light supply means which supplies a first pumping light to excite the first optical fiber, and a non-reciprocal means which controls the reflection of light on the input port and the output port. The first gain equalizers comprise a second optical fiber for flattening the gain of the power of the optical signal to be emitted from the output port within a fixed wavelength range. According to this composition, flattening of the output gain of the first and second main optical amplifiers can be achieved by the first gain equalizers. For this case, a function can be provided in the non-reciprocal means itself that cuts out pumping light. Alternatively, in addition to the above-mentioned non-reciprocal means, a separate means that cuts out pumping light may be provided as necessary.
According to a third aspect of the present invention, a method is provided that uses optical fiber amplifiers to amplify a wavelength division multiplexing signal that enters from an input port and is emitted from an output port to flatten the power of the wavelength division multiplexing signal to be emitted.
This flattening method comprises first and second optical fibers connected in series to the optical fiber amplifier. The first optical fiber is actuated by wavelength dependency amplification characteristics. The second optical fiber is actuated by wavelength dependency absorption characteristics. And, a wavelength division multiplexing signal input to the first optical fiber undergoes batch amplification from the first optical fiber and then the power of the wavelength division multiplexing signal is flattened using wavelength dependency absorption characteristics of the second optical fiber.
According to this composition, investigations were carried out in advance by means of experimentation on wavelength dependency amplification characteristics of the first optical fiber and wavelength dependency absorption characteristics of the second optical fiber with characteristics opposite to the first optical fiber. Then, gain of the optical signal to be emitted from the output port could be flattened in a fixed wavelength region of the wavelength division multiplexing signal that enters the input port and within a fixed power region using the second optical fiber to compensate for output deviations of the first optical fiber.
In a preferred embodiment of the method of the present invention, a wavelength division multiplexing signal output from the second optical fiber passes through the first optical fiber is sent to the output port.
In another preferred embodiment of the method of the present invention, a wavelength division multiplexing signal that enters from the input port passes through the first optical fiber is sent to the second optical fiber.
In this embodiment of the method of the present invention, it is preferable for the wavelength division multiplexing signal to pass through the second optical fiber while the second optical fiber is in a non-pumping state or in the absorption region.
Furthermore, in this embodiment of the method of the present invention, it is preferable for the wavelength division multiplexing signal to pass through the second optical fiber while the second optical fiber is in an pumping state within a range in which an optical signal is not amplified. Or, the composition can be such that the pumping state of the second optical fiber changes in response to the state of the power of the wavelength division multiplexing signal.
Thus, by means of exciting the second optical fiber or making it possible to adjust that pumping state within a range in which an optical signal is not amplified, the gain of the output power can be flattened with even more accuracy.